Battery gauges or monitors include electronic circuits that estimate the current power level or capacity of batteries. An estimate of the current power level or capacity can be generated based, in part, on measuring the voltage provided by a battery. For example, if the battery provides a relatively large supply voltage, then the supply voltage can indicate that the battery has a significant portion of capacity remaining. Estimates of the remaining capacity of the battery can be useful for optimizing the system efficiency or regulating the supply power. Moreover, the remaining capacity is relevant information for applications that draw power from the battery, as well as applications that charge the battery.